Bump minimum supported version to Android 9

For Qt 6.8.0, the current Android version that matches the 90% range in https://apilevels.com would be Android 9, and that's the planned version to bump to for the upcoming 6.8.0 release.

The bump needs be reflected in the official docs, in CI tests and in the build system.

This was the outcome from https://lists.qt-project.org/pipermail/development/2024-April/045212.html